Abstract Defects in antigen presenting cell (APC) function and immunoregulation underlie the development of type I diabetes in NOD mice and humans. Studies have shown that CD4+CD25+ regulatory T cell function is reduced in NOD mice and diabetic patients. Our preliminary studies suggest the defect resides in the ability of NOD APC to activate CD4+CD25+ regulatory T cells rather than in the CD4+CD25+ regulatory T cells themselves. We hypothesize that a defect in the APC compartment in NOD mice may result in deficient regulatory T cell activation and may contribute to diabetes development. Infection or treatment with a number of different types of microorganisms, including lactobacillus and M. tuberculosis, prevent or delay diabetes in NOD mice. Interestingly, microorganisms can evade the immune response by inducing tolerance through the targeting of Toll-like receptors (TLR) and/or C-type lectin receptors (CLR) expressed by APC. For this reason, we propose that the microorganisms that prevent diabetes act on NOD APC via TLR and/or CLR, enabling the APC to activate/induce regulatory T cells. The objective of the present proposal is to determine whether TLR/CLR ligands can enhance NOD APC ability to activate regulatory cells, and prevent or treat diabetes. In Aim 1, we will analyze the defect in NOD APC. In Aim 2, we will evaluate the mechanisms by which Lactobacilli prevent disease in NOD mice. In Aim 3, we will examine whether TLR2 and/or CLR ligands that have been associated with tolerance induction can enhance NOD APC activation of CD4+CD25+ regulatory T cells and prevent disease in mice. The results of these experiments should provide a basis for the development of novel strategies for the treatment of diabetes.